From Cabin Lights to Water Pumps
Calculating Your Solar Load Needs
- Introduction
- A Cabin Walkthrough
- What is a Load List and Why Is It Essential?
- Finding Power Ratings for Your Devices
- Example Load Calculations
- Total Daily Energy Consumption
- Conclusion
Introduction
When I work with cabin owners designing new solar systems or upgrading older setups, we always begin with a close look at their power needs. Many cabins already have systems, but often, these systems are outdated, or the owner is ready for more capacity to support new appliances and larger loads. Today, I’m putting into words what a typical cabin owner might think about when they first walk into their new cabin, envisioning a solar setup to support their lifestyle.
Before getting into the technical aspects of design, we always start with a fundamental step: the load list. This list includes every device and appliance they plan to use, along with the expected daily power consumption. It’s a crucial foundation because it guides every part of the system design, ensuring it meets their energy needs and helps avoid unnecessary upgrades in the future. So, let’s break down what this initial load list entails and how it shapes the future of a reliable, efficient off-grid system.
A Cabin Walkthrough
I start by walking through each room, mentally adding up the potential loads. I imagine the LED lightbulbs that will softly light up the cabin, providing an energy-efficient solution for evenings here. Next, I think of essential items like the water pump, which will supply fresh water, and the TV, promising cozy evenings of entertainment. Practical necessities come to mind as well, such as a small laptop for work and of course my cell phone for communication.
With each device noted, I add a rough estimate of its daily usage, imagining how my day-to-day routines will translate into energy consumption. This process gives me a clearer picture of the total energy I’ll require and provides insight into the size of the battery bank needed to power these devices consistently, even when the sun isn’t shining.
This initial assessment lays the groundwork for my off-grid solar system design. It’s a necessary first step, allowing me to envision a self-sufficient lifestyle that aligns with my values of sustainability and independence. With each load calculated and each hour estimated, I feel more prepared to turn this cabin into a true home, one that runs on the energy of the sun.
What is a Load List and Why Is It Essential?
A load list is a catalog of all the electrical devices you plan to use, including each one’s power rating and estimated daily usage. This information allows you to calculate your total daily energy consumption in watt-hours (Wh), which in turn guides the design of your solar system’s size and configuration. Every piece of equipment in an off-grid setup, from the batteries and solar panels to the inverter, depends on knowing exactly how much power you’ll use on an average day.
Finding Power Ratings for Your Devices
Creating a load list begins with identifying the power consumption of each device. Most electrical devices, appliances, and lightbulbs have labels indicating their power requirements, typically measured in watts (W) or amps (A). These labels are usually found on the back or underside of the device, or sometimes on the packaging. Here’s how to interpret them:
Direct DC Usage from Battery (No Inverter)
When a device runs directly from a battery at, say, 12 volts DC, you calculate its wattage by multiplying the amps (A) by the voltage (V):
- Example: A water pump labeled 5 amps at 12 volts. 5 A × 12 V = 60 watts (W)
- If the pump runs for 2 hours: 60 W × 2 hours = 120 watt-hours (Wh)
This 120 Wh is directly drawn from the battery without any additional loss.
AC Usage Through an Inverter
If you’re using an inverter to power an AC device (like a 120V lightbulb), the calculation is similar, but remember the inverter pulls power from the DC battery and converts it to AC for household-type devices.
- Example: A 120V AC LED lightbulb rated at 8.5 watts. Even though this light uses 8.5 watts in AC power, the inverter draws DC energy from the battery to provide that 8.5 watts.
- If you use it for 3 hours: 8.5 W × 3 hours = 25.5 Wh
Although inverters have some efficiency loss, this estimate gives you a rough idea of how much energy the lightbulb will use from your battery, allowing you to build an energy-use profile for your load list.
By using this method to calculate the daily watt-hours for each device, you can create a detailed load list that accurately estimates your energy needs. With this information, you’re ready to start planning the ideal solar system to meet your cabin’s requirements.
Example Load Calculations
To illustrate, let’s consider a few devices you might want to use in your cabin:
LED Lightbulbs (8.5 Watts each, AC):
I find the wattage labeled usually right when you purchase them, they should say right on the packaging the wattage of the bulb.
- Devices: You plan to install three of these.
- Daily Use: One light will be on for 3 hours, while the other two will each run on and off for about 30 minutes.
- Calculation:
- 1 light: 8.5 𝑊 × 3 hours = 25.5 Wh
- 2 lights: 8.5 𝑊 × 0.5 hours × 2 = 8.5 Wh
- Total: 25.5 Wh + 8.5 Wh = 34 Wh per day
Water Pump (5 Amps at 12 Volts, DC):
Most pumps and motors include labels that display essential specifications. If the label is worn or unreadable, try locating a model number on the device, which you can often look up online to retrieve the exact specifications originally listed on the label. Here below we found our model number and the energy it uses in Amps at 12 volts.
- Device: A DC water pump for cabin water supply.
- Power Rating: 5 amps at 12 volts = 60 watts.
- Daily Use: 1 hour.
- Calculation:
- 60W × 1hour = 60Wh
- Total: 60 Wh per day
- Note: Using a DC pump directly from the batteries eliminates the need for an inverter, which improves efficiency as there’s no loss due to conversion from DC to AC power.
TV (200 Watts, AC):
TV’s have their labels display their specifications usually imprinted directly on the plastic where it never wears out
- Device: A small TV for evening entertainment.
- Daily Use: 3 hours.
- Calculation:
- 200W × 3hours = 600Wh
- Total:600 Wh per day
Laptop (45 Watts, AC):
- Device: A laptop for work or leisure.
- Daily Use: 2 hours to charge, after which it can run on its internal battery for an additional hour.
- Calculation:
- 45W × 2hours = 90Wh
- Total:90 Wh per day
Cell Phone Charger (5 Watts, AC):
- Device: A cell phone charger
- Daily Use: 0.5 hours (30 minutes)
- Calculation:
- 5 𝑊 × 0.5 hours = 2.5Wh
- Total: 2.5 Wh per day
Total Daily Energy Consumption
By adding up the estimated usage for each of these devices, you get a picture of your cabin’s daily energy needs:
- LED Lights: 34 Wh
- Water Pump: 60 Wh
- TV: 600 Wh
- Laptop: 90 Wh
- Cell Phone: 2.5 Wh
- Total:786.5 Wh per day
786.5 Wh
So, your cabin’s anticipated daily energy usage would be 786.5 Wh per day.
Your cabin’s estimated daily energy usage comes to 786.5 Wh. It’s important to note, however, that the water pump’s power draw, at 60 Wh, is handled directly from the DC battery and doesn’t pass through the inverter like the other appliances. Since it runs on DC, it avoids the conversion to AC power, making it a bit more efficient by sidestepping any losses from the inverter.
For simplicity, we’re basing these calculations on a 12-volt system. Once you’ve completed your load list and have an estimate of your energy consumption, you’ll have the data needed to choose your system voltage effectively. But for now, focusing on the total energy consumption is the essential first step in building a system tailored to your needs.
Conclusion
This daily energy estimate is your foundation for understanding how much stored power you’ll need to keep everything running smoothly. As you first walk through your new cabin, imagining a future solar system, take a close look at everything around you. Consider each appliance, light, and device you’d like to power, jotting it all down as you go.
While this example focused on just a few devices, your load list will likely include more, helping ensure all your energy needs are accounted for. And as we plan for uninterrupted power, it’s wise to factor in days of autonomy, times when sun exposure is limited, like during a three-day cloudy spell, by multiplying your total daily watt-hours accordingly.
This is just the beginning of the off-grid journey. Once all loads are identified and tallied, we’ll get into the next steps, starting with battery selection to ensure your system has the energy storage it needs. If you’re excited to continue learning, stay tuned for more insights as we work through each stage of building a resilient, off-grid energy solution.
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